1. Field of the Invention
The present invention relates generally to digital-to-analog converters and, more particularly, to multiplying digital-to-analog converters.
2. Description of the Related Art
Digital-to-analog converters (DACs) convert digital input signals to corresponding analog output signals. In a typical DAC embodiment, an internal reference signal (e.g., a reference current) is mirrored to form a plurality of currents which flow through switches that respond to a digital input signal. The combined currents thus form an analog output current signal (which can, if desired, be converted to a voltage output signal) that has thereby been converted from the digital input signal.
In a multiplying digital-to-analog converter (MDAC), an input signal Sin is received so as to modify the internal reference signal and thus the input signal Sin is converted to an analog output signal Sout with a conversion gain that corresponds to the digital signal which is now considered to be a digital command signal Scmd.
MDACs have been found useful for a variety of applications (e.g., as the xe2x80x9cattenuation corexe2x80x9d in coaxial cable line drivers). However, it has also been found that conventional MDAC structures generally introduce distortion components into the analog output signal Sout. Although it has also been found that the distortion can be reduced by increasing current levels throughout the MDACs and by adding transistor current drivers (e.g., source followers), the first alteration decreases efficiency and raises component heating and the second has realized only moderate improvement.
The present invention is directed to MDACs that reduce signal distortion without significantly raising current demand. These goals are achieved with input structures that lower input impedances and enhance the driving of nonlinear capacitances that are generally presented by the DAC portion of these devices.